Magnetic chucks and worktables, of the type with which the present invention is concerned, are commonly used to retain a workpiece during machining or other processing, the workpiece being composed generally of a ferromagnetic material or having magnetically attractable properties. In general such magnetic chucks and worktables, hereinafter referred to as magnetic retention plates for selectively retaining a magnetically attractable body, can comprise permanent magnets and an actuating mechanism which can shift pole pieces relative to the magnets, magnets relative to one another or other combinations of magnets, magnetically permeable members or magnetically insulating members to create a state at which a magnetic field appears at the surface of the magnetic retention plate to hold the body against this surface, or this magnetic field is nullified or attenuated to allow removal and replacement of this body on the surface.
In this connection mention may be made of the commonly owned U.S. Pat. Nos. 3,775,717, 4,379,277, 4,465,993, and 4,542,890 and the documents cited in the files thereof.
Of particular interest in the present invention is a magnetic retention plate of this type which comprises above a base plate or sole plate, an array of pole pieces separated by partitions so that the surface of the retention plate adapted to receive the magnetically attractable body is defined by the ends of the pole pieces and the edges of the intervening separators or partitions.
An array of fixed magnets magnetized in a common direction and an array of movable permanent magnets magnetized in a direction perpendicular to the aforementioned common direction are also provided.
The actuating mechanism can displace the array of movable magnets in translation between a first position in which the lines of force from all of the magnets are closed within the magnetic plate, and a second position in which the lines of force of all the magnets through the pole pieces can only close externally of the magnetic plate.
This particular type of magnetic retention plate is utilized to hold workpieces to be machined on the table of a machine tool, the permanent magnets creating at least one active face of the plate in the second position with an attractive force capable of retaining the workpiece against this face.
This type of magnetic plate in which the direction of magnetization of the movable magnets is perpendicular to the direction of magnetization of the fixed magnets is described in French patents No. 2,017,322 (see also U.K. patent specification No. 1,274,533) and No. 2,536,321 (see U.S. Pat. No. 4,468,649).
In the magnetic retention plates of this type which have been developed heretofore, the pole pieces and the partitions or separators only form a fraction of the active face of the plate in the longitudinal direction, each pole piece and separator extending transversely over the entire width of the magnetic plate.
The magnetic attraction, while excellent in one direction, generally is comparatively poor in a direction perpendicular thereto and thus one must be careful how the workpiece is positioned on the magnetic chuck.
Furthermore, if the article is positioned improperly on the magnetic chuck, it may not be held securely and may create a danger to the machinist, create a problem of machining accuracy, or result in damage to an expensive workpiece.